Distinguishing false signals in cable locating

ABSTRACT

Discriminating between a cable locating signal and a false cable locating signal is described. A reference signal, which contains a locating signal frequency impressed on it, is transmitted in a way which provides for detection of a phase shift between the locating signal and the false locating signal. Based on the phase shift, a receiver is used to distinguish the locating signal from the false locating signal.

RELATED APPLICATIONS

The present application is a continuation application of co-pending U.S.patent application Ser. No. 13/301,487 filed on Nov. 21, 2011; which isa continuation of U.S. patent application Ser. No. 12/979,793 filed onDec. 28, 2010 and issued as U.S. Pat. No. 8,080,998 on Dec. 20, 2011;which is a continuation of U.S. patent application Ser. No. 12/512,853filed on Jul. 30, 2009 and issued as U.S. Pat. No. 7,884,610 on Feb. 8,2011; which is a continuation of U.S. patent application Ser. No.12/137,424 filed on Jun. 11, 2008 and issued as U.S. Pat. No. 7,586,308on Sep. 8, 2009; which is a continuation of U.S. patent application Ser.No. 11/557,432, filed Nov. 7, 2006 and issued as U.S. Pat. No. 7,397,249on Jul. 8, 2008; which is a continuation application of U.S. patentapplication Ser. No. 11/105,804, filed on Apr. 13, 2005, and issued asU.S. Pat. No. 7,151,375 on Dec. 19, 2006; the disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a system and method forlocating buried lines such as, for example, pipes, cables, conduits orother conductors and, more particularly, to a locating arrangement thatis configured for distinguishing “false” cable locating signals.

The installation of utility lines underground is increasingly popularfor reasons of aesthetics and for practical reasons such as, forexample, protecting these lines from the effects of severe above groundweather conditions. In areas where buried lines have previously beeninstalled, however, it is often necessary to locate the position of thelines. This task is generally accomplished by causing the line to emitan electromagnetic locating signal by connecting a transmitterarrangement to an electrically conductive component of the line. It isnoted that such an electrically conductive component may be providedspecifically for this purpose such as, for example, a tracer wire aspart of a fiber optic cable. Once the location of the cable or cables ina region has been identified, subsequent operations such as excavationin the region may be performed in a way that avoids pre-existing cablelocations. Of course, sufficient locating accuracy is a necessity.

Unfortunately, the prior art has been limited by at least one aspect ofthe aforedescribed technique. Specifically, the electromagnetic signalthat is transmitted from a particular cable of interest can be coupledto another cable in the region. This coupling can take place through theground or can occur when the cables are in physical contact, forexample, at their distal ends whereby a direct, resistance-typeelectrical connection is formed between the cables. Once the locatingsignal is coupled to another cable, that cable will re-emit the locatingsignal. Using prior art systems, the re-emitted locating signal is, inmost cases, indistinguishable from the original locating signal. Whenfaced with this situation, it is not uncommon for an operator of anabove ground locator to follow a cable that is re-emitting the locatingsignal, with no indication that a different cable is, in fact, beinglocated. Thus, a considerable amount of confusion is possible when morethan one cable is present in the operating region.

The present invention resolves the foregoing limitations in a highlyadvantageous way, while providing still further advantages, as will bedescribed immediately hereinafter.

SUMMARY OF THE DISCLOSURE

In a region which includes at least first and second in-ground cablesextending across the region, each of which cable includes anelectrically conductive component such that, when the first cable isdriven at a locating signal frequency to emit a locating signal in anelectromagnetic form, the locating signal is coupled to the second cablein a way which causes the second cable to generate a false locatingsignal in electromagnetic form, a system and method are described fordiscriminating between the locating signal and the false locatingsignal. In one aspect of the invention, a reference signal, whichcontains the locating signal frequency impressed thereon, is transmittedin a way which provides for detection of a phase shift between thelocating signal and the false locating signal. Based on the phase shift,a receiver is used to distinguish the locating signal from the falselocating signal.

In another aspect of the invention, the locating signal iselectromagnetically coupled from the first cable to the second cablethrough the ground to cause the second cable to emit the false locatingsignal. The locating signal frequency is recovered from the referencesignal. Synchronous detection of the locating signal frequency and aselected frequency band, which includes said locating signal and saidfalse locating signal, is performed in a way which substantially rejectsthe false locating signal while providing an output responsive to thelocating signal.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The present invention may be understood by reference to the followingdetailed description taken in conjunction with the drawings brieflydescribed below.

FIG. 1 is a diagrammatic elevational view of a region in which the cablelocating system and method of the present invention are operatingincluding two cables disposed in the region, one of which transmits thelocating signal of interest and the other of which transmits a false,potentially interfering signal.

FIG. 2 is a diagrammatic plan view of the region of FIG. 1 including thecables disposed therein along with the system of the present invention.

FIG. 3 is a diagrammatic plan view of the region in which the cablelocating system and method of the present invention are operatingincluding two cables that are resistively connected at distal endsthereof.

FIG. 4 is a block diagram which illustrates one embodiment of atransmitter produced in accordance with the present invention.

FIG. 5 is a diagrammatic see-through view, in perspective, whichillustrates one embodiment of a locator/receiver that is produced inaccordance with the present invention.

FIG. 6 is a diagrammatic see-through view, in perspective, whichillustrates another embodiment of a locator/receiver of the presentinvention including a dependent wand which houses an additional antennaarrangement.

FIG. 7 is a block diagram which illustrates one embodiment of a receiverthat may be used in a portable locator in accordance with the presentinvention.

FIG. 8 is a block diagram which illustrates another embodiment of atransmitter, produced in accordance with the present invention,including selective cable locating signal connection capabilities.

FIG. 9 is a block diagram which illustrates still another embodiment ofa transmitter, produced in accordance with the present invention, thatis capable of transmitting a reference signal using an RF carrier.

FIG. 10 is a block diagram which illustrates another embodiment of areceiver/locator of the present invention which is configured forreceiving a reference signal modulated on an RF carrier.

FIG. 11 is a diagrammatic view, in elevation, of a region in which thetransmitter of FIG. 9 and the receiver of FIG. 10 are in use, shown hereto illustrate further details with respect to their operation.

DETAILED DESCRIPTION

The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe described embodiments will be readily apparent to those skilled inthe art and the generic principles herein may be applied to otherembodiments. Thus, the present invention is not intended to be limitedto the embodiment shown but is to be accorded the widest scopeconsistent with the principles and features described herein includingalternatives, modifications and equivalents, as defined within the scopeof the appended claims. It is noted that the drawings are not to scaleand are diagrammatic in nature in a way that is thought to bestillustrate features of interest. Further, like reference numbers areapplied to like components, whenever practical, throughout the presentdisclosure. Descriptive terminology such as, for example, upper/lower,right/left, front/rear may be employed for purposes of enhancing thereader's understanding, with respect to the various views provided inthe figures, and is in no way intended as being limiting.

Turning now to the drawings, attention is immediately directed to FIG. 1in which a locating region is generally indicated by the referencenumber 10 and shown in a diagrammatic elevational view. Region 10includes a first cable 12 and a second cable 14, each of which includesan electrically conductive component which may be used for purposes oftransmitting an electromagnetic locating signal, as will be furtherdescribed. The surface of the ground is indicated by reference number16.

Referring to FIG. 2 in conjunction with FIG. 1, the former is adiagrammatic plan view of region 10. The ground is illustrated as beingtransparent with the exception of its surface 16 in FIG. 1. The highlyadvantageous cable locating system of the present invention isillustrated in each of FIGS. 1 and 2, generally indicated by thereference number 100. System 100 includes a transmitter 102 and areceiver/locator 104, as will be described in detail at appropriatepoints hereinafter. For the moment, however, it is sufficient to notethat transmitter 102 generates a locating signal frequency f_(L) and areference signal R, and includes any suitable coupling configuration 112for coupling the locating signal frequency or the locating frequency incombination with the reference signal onto cable 12. This coupling canbe accomplished using inductive coupling or a direct electricalconnection, both of which are considered as familiar to one havingordinary skill in the art. It is noted that exemplary waveforms havebeen illustrated in FIG. 1 with respect to f_(L) and R as well as otherphysical quantities of interest, as will be further described.

Having coupled the locating signal frequency and the reference signalonto cable 12, this cable emanates a locating signal which is designatedas L and a signal R′ which designates the reference signal subject tothe phase shift of coupling configuration 112. It should be appreciatedthat the same phase difference is introduced between locating signalfrequency f_(L) and locating signal L. The specific magnitude of thephase shift depends upon the specific type of arrangement that is usedto couple the locating signal frequency to the cable. In the presentillustrations, the use of inductive coupling is contemplated whereinlocating signal L leads locating signal frequency f_(L) by approximately90°. For reasons which will be made evident below, an awareness of thephase characteristics of coupling configuration 112 is generally needed.

As described above with respect to the prior art, locating signal Ltravels through the ground and can be coupled to cable 14. Of course,the degree of coupling is dependent upon various factors such as, forexample, proximity and resistive path from one cable to the other. Adetailed discussion of these factors, however, is not necessary to anunderstanding of the present invention and is therefore considered to bebeyond the scope of the present description. What is important tounderstand is that cable 14, under many circumstances, produces areemitted or “false” locating signal L_(F). If this reemitted locatingsignal is viewed solely in terms of its frequency, it isindistinguishable from locating signal L. Hence, many operators of priorart cable locating systems may inadvertently follow cable 14, as opposedto cable 12. Moreover, during a particular locating session, theoperator may inadvertently switch between these two cables, yielding anextremely confusing picture of what appears to be the actual undergroundconfiguration of the cable. It should also be appreciated that there maybe any number of “secondary” cables within the region in addition to the“target” cable, although only one secondary cable has been illustratedin the present example for purposes of clarity. Each one of thesecondary cables can potentially produce an interfering locating signal.The composite of all of these interfering signals can make it next toimpossible to identify the target cable of interest. As will bedescribed immediately hereinafter, the present invention sweeps asidethese difficulties.

Turning to FIG. 3, a diagrammatic plan view of region 10 illustrates analternative situation with respect to a first cable 12′ and a secondcable 14′. In particular, each of these cables includes a distal end andthese distal ends are electrically connected to one another by aresistance connection. The latter may occur deliberately, for example,in a junction box (not shown) or through inadvertent direct contact. Thespecific mechanism through which the resistance connection arises has noeffect on system 10 and the use of the locating technique to bedescribed in further detail below.

Having described region 10 in detail above, attention is now directed todetails with respect to the configuration of the locating system withreference to FIG. 4, which illustrates one embodiment of transmitter 102in the form of a block diagram. Details with respect to the variousfunctional blocks have not been provided, since it is considered thatthe provision of circuitry for satisfying these functions is within thecapability of one having ordinary skill in the art, as is also the casewith all block diagrams of the present disclosure. Transmitter 102includes a high-frequency or carrier generator 140 which may operate atany suitable frequency in view of a contemplated locating signalfrequency (as illustrated in FIG. 1). The latter is generated by alow-frequency or locating signal frequency generator 142. One usefullocating signal frequency has been found to be approximately 30 kHz witha carrier generator frequency of approximately 300 kHz, although thesevalues are not intended to be limiting. By way of example, the locatingsignal frequency may be in the range of approximately 1 Hz to 80 KHzwhile the carrier generator frequency may be in the range ofapproximately 10 Hz to 300 KHz. In this regard, it may be useful tomaintain a value in of from 5 to 10 as a ratio of the carrier generatorfrequency to the locating signal frequency. In this regard, no harmonicrequirement need be imposed with respect to this frequency relationship.A modulator 144 receives the carrier generator signal from carrierfrequency generator 140, as well as the locating signal frequency fromlow frequency signal generator 142. Modulator 144 impresses the locatingsignal frequency onto the carrier frequency using any suitable form ofmodulation such as, for example, amplitude modulation or frequencymodulation. An output 146 of modulator 144 comprises a modulatedcarrier, the appearance of which depends on the specific form ofmodulation that is employed, as will be familiar to those havingordinary skill in the art. This modulated output may be referred toherein as a “reference” signal for reasons which will become evidentbelow. It is noted that the reference signal corresponds to R of FIG. 1.Output 146 is connected to a summer 148. Locating signal generator 142also provides its output to summer 148. An output of summer 148comprises the additive combination of the reference signal and thelocating signal frequency, R+f_(L). This combination is then supplied toan amplifier which drives coupling configuration 112. As describedabove, the latter may comprise an inductive clamp, an inductivearrangement (whereby the transmitter is simply positioned immediatelyadjacent to the cable), or a direct electrical connection.

Referring to FIGS. 1, 2 and 4, the combination of R+f_(L) is induced oncable 12 by transmitter 102. A corresponding signal that is emanatedfrom cable 12 is indicated as K+L, in FIG. 1. It is noted that only awaveform corresponding to L has been shown in FIG. 1 for purposes ofillustrative clarity. Further, a (′) prime mark, applied to thereference signal, designates that the signal is subject to a phase shiftby virtue of passing through coupling configuration 112. In theembodiment presently under discussion, it is to be understood that R′appears as essentially identical to R, as will be further discussed.With respect to L, however, it is apparent that a phase shift hasoccurred when compared with f_(L). Generally, this phase shift will beapproximately 90 degrees, depending upon coupling configuration 112. Inthe present example, L is shown as leading f_(L). It is recognized that,while the carrier frequency of reference signal R′ is shifted in thesame manner as locating signal L, the modulation envelope of thereference signal is unaffected, at least from a practical standpoint.That is, the modulation envelope of phase shifted reference signal R′ isessentially identical to the modulation envelope of reference signal Rand the phase shift of the carrier produces only a small phase shift ofthe modulation envelope which would affect determinations made hereinthat rely on the modulation envelope. Thus, locating signal frequencyf_(L) is recoverable from R′ as its modulation envelope. As will beseen, this recognition is applied in a highly advantageous manner uponreception of the various signal components.

Turning to FIG. 5, one embodiment of portable locator 104 is illustratedand configured in accordance with the present invention. Locator 104includes a three-axis antenna cluster 202 measuring three orthogonallyarranged components of magnetic flux in locator fixed coordinates. Theantenna cluster could be composed of magnetometers which would beadvantageous at lower frequencies. One useful antenna clustercontemplated for use herein is disclosed by U.S. Pat. No. 6,005,532entitled ORTHOGONAL ANTENNA ARRANGEMENT AND METHOD which is commonlyassigned with the present application and is incorporated herein byreference. It is noted that this antenna cluster is advantageous withrespect to generating a vector sum of the local flux intensity. Antennacluster 202 is connected to a receiver section 204, which will bedescribed in detail at an appropriate point below. A tilt sensorarrangement (not shown) may be provided for measuring gravitationalangles from which the components of flux in a level coordinate systemmay be determined. Locator 200 further includes a graphics display 206,a keypad 208 and a microprocessor 210 interconnected appropriately withthe various components. A telemetry arrangement 212 having an antenna214 is optional, under control of microprocessor 210, and is providedbased on the specific system implementation, as will be discussed atappropriate points hereinafter. Other components may optionally be addedsuch as, for example, an azimuth sensor in the form of a tri-axialmagnetometer 216 to aid in position determination relative to a selecteddirection such as a cable orientation and ultrasonic transducers (notshown) for measuring the height of the locator above the surface of theground. One highly advantageous ultrasonic transducer arrangement isdescribed, for example, in U.S. Pat. No. 6,232,780 which is commonlyassigned with the present application and is incorporated herein byreference.

Still referring to FIG. 5, antenna cluster 202 is oriented having areceiving axis arranged along an axis of symmetry of the locator. Inparticular, components of the received electromagnetic locating signalare designated as b_(x),b_(y),b_(z) with b_(x) being oriented along theaxis of symmetry of the locator. It is noted that locator 104 is usefulwith respect to a wide variety of cable locating techniques. One highlyadvantageous cable locating technique is described in U.S. Pat. No.6,496,008, entitled FLUX PLANE LOCATING IN AN UNDERGROUND DRILLINGSYSTEM, which is commonly owned with the present application and isincorporated herein by reference. Additional highly advantageous cablelocating techniques are described in U.S. Pat. No. 6,737,867 which iscommonly owned with the present application and incorporated herein byreference. It should be appreciated that the teachings herein areconsidered to provide remarkable advantages, irrespective of the detailsof the specific locating technique that is employed, since virtually allof these cable locating techniques can be improved with the capabilityto reject unwanted interference. Moreover, as will be seen, thedescribed technique operates in a manner which is most often transparentto the operator of locator 102.

Referring to FIG. 6, a modified locator 104′ is illustrated. Locator104′ is essentially identical to locator 104 with the addition of adownwardly dependent wand 250. The latter includes a distal end whichhouses an additional antenna cluster 202′. Details with regard to theuse of this alternative locator implementation will be provided at anappropriate point below.

Turning to FIG. 7, one embodiment of locator 104 is shown here toillustrate additional details with respect to its features. Inparticular, controls 270 may include items such as, for example, anoperator actuatable trigger or push button switch. It is noted that thisparticular embodiment does not use a telemetry arrangement. Receiversection 204 includes a receiver and signal processing section 271 and ahigh frequency section 272, connected to section 271, having a bandpassfilter that passes phase shifted reference signal R′. The high frequencysection provides this output to a demodulator 274 in order to recoverlocating signal frequency f_(L) from the reference signal. The locatingsignal frequency is then fed to a phase shifter 276. The latter is undercontrol of processor 210 via a control line 277. Phase shifter 276 iscontrolled, depending upon the particular phase shift characteristics ofcoupling configuration 112, to provide a phase shift that is essentiallyequal to the phase shift that is introduced by coupling configuration112. Hence, the output of phase shifter 276 is designated as f′_(L) or,in other words, the locating signal frequency with a phase shift thatmatches the coupling configuration phase shift, so as to be in phasewith the locating signal. Moreover, phase shifter 276 can additionallyprovide compensation for circuitry induced phase shift, if needed, tocause f′_(L) to be in phase with the locating signal. It is noted thatphase shifter 276 may not be necessary in some implementations, forexample, when the locating signal and reference signal areresistively/directly coupled to the cable or associated tracer wire.Alternatively, as will be described below, another embodiment of thetransmitter may include phase shift compensation.

Still referring to FIG. 7, a synchronous detector 278 includes twoinputs, one of which receives shifted locating signal frequency f′_(L).The other input of the synchronous detector is connected to a lowfrequency section 280, which is itself connected to antenna cluster 202via receiver and signal processing section 271. The low frequencysection thus provides a band limited output to the other input ofsynchronous detector 278 which contains locating signal L along withfalse locating signal L_(F). Hence, the synchronous detector provides astrong output responsive to the combination of f′_(L) and L, since thesetwo signals are in phase, thereby essentially passing the locatingsignal on to processor 210. False locating signal L_(F), on the otherhand, is rejected since it is, at least approximately, 90 degrees out ofphase with shifted locating signal frequency f′_(L). That is, thequadrature component of the false locating signal can be rejected bysynchronous detection. Processor 210 then uses the received relativesignal strength of the locating signal according to its intendedlocating procedures. As mentioned above, rejection of the false locatingsignal is essentially transparent to an operator of the locator.

Referring briefly to FIG. 3, in this example, the resistance connectionbetween cables 12′ and 14′ results in false locating signal L_(F) being180 degrees out of phase with respect to locating signal L, asillustrated. For this reason, if the operator switches from cable 12 tocable 14 during locating, and while at least generally maintaining theorientation of the locator, the signal strength shown on display 206(FIG. 5) will change signs, thereby notifying the operator that he orshe is no longer following cable 12′.

Turning again to FIG. 6, since antenna cluster 202 can be used toascertain an essentially simultaneous signal strength along each of itsthree orthogonal receiving axes, a total field strength can bedetermined in the form of a vector sum. Further, antenna 202′ can alsobe used to determine an additional total signal strength at itslocation, spaced apart from cluster 202. It is recognized that thisconfiguration is particularly advantageous, since the use of both vectorsums reduces orientation requirements with respect to the way in whichthe operator orients the locator with respect to the cable. Morespecifically, the vector sum will produce the same value for anyorientation of the locator including parallel, perpendicular orotherwise with respect to the cable. The use of a vector sum withrespect to one antenna cluster is described in U.S. Pat. No. 5,155,442,which is commonly owned with the present application and is incorporatedby reference. Hence, the use of two vector sums is considered as avariation that is within the capability of one having ordinary skill inthe art. In particular, the locating field over the cable is normallyassumed to be a circular field with all of the horizontal projections ofthe fluxlines perpendicular to the cable. With two antennas, the upperand lower antennas are used to obtain a ratio of the field strengths attwo elevations in order to calculate depth. This ratio is the same,whether using just a portion of the field, with equal misalignment ofthe antennas, or using the total field strength.

Referring to FIG. 8, another embodiment of a transmitter, that isconfigured in accordance with the present invention, is generallyindicated by the reference number 102′. Comparison with FIG. 4 revealsthat transmitter 102′ is similar in structure to previously describedtransmitter 102. Hence, the present discussion will be limited todifferences between these transmitters. One difference resides in theuse of a phase shifter 276′ in transmitter 102′, which eliminates theneed for the use of a phase shifter in a receiver which cooperates withtransmitter 102′. Of course, if the receiver includes a programmablephase shifter, it can be set to introduce zero phase shift. Further, amodified coupling arrangement 112′ has been provided which includes aninductive clamp 300, a coupling antenna 302 and a direct or resistiveclip 304 each of which may be used selectively. In use, the inductiveclamp is clamped around the cable in a well known manner; couplingantenna 302 is positioned adjacent to the cable onto which it is desiredto induce a signal; and clip 304 is attached directly to the cable ortracer wire. A switching arrangement 310 is connected between amplifier152 and each selectable coupling arrangement. In this regard, it shouldbe appreciated that an inductive configuration, such as is provided byclamp 300 and antenna 302, will introduce a phase shift, while directclip 304 will not. Hence, switching arrangement 310 outputs a selectionsignal to phase shifter 276′, as indicated by a dashed line 312, in away which causes phase shifter 276′ to introduce an appropriate phaseshift, based on the particular selection of coupling that is made usingswitch 310. It should be appreciated that the latter can be configuredin a wide variety of ways in view of this overall disclosure. Forexample, three separate input jacks can be provided, each of which iscustomized for one type of coupling connection whereby connection to aparticular jack produces the appropriate selection signal on line 312.As another example, a rotary or push button switch can be provided toelectronically select the type of coupling that is in use.

Attention is now directed to FIG. 9, which illustrates anotherembodiment of the transmitter of the present invention, generallyindicated by the reference number 102″. In this case, low frequencygenerator 142 provides locating signal frequency f_(L) directly toamplifier 152 which remains connected to coupling configuration 112′. Atthe same time, the locating signal frequency is also provided to phaseshifter 276′, which remains responsive to switching arrangement 310, asdescribed above. Phase shifter 276′ outputs phase shifted locatingsignal f′_(L) to modulator 144. Again, it should be remembered thatphase shifter 276′ is not required, for example, when a phase shifter isprovided in a cooperating receiver.

Transmitter 102″ also includes an RF generator 320 having an output 322which provides an RF carrier to modulator 144. In this way, modulator144 generates reference signal R of FIG. 1. The reference signal iscoupled to an RF amplifier 324 which drives an RF antenna 330 so as toemanate reference signal R. As described above with respect to thereference signal, any suitable form of modulation may be utilized. Inthis particular embodiment, however, an RF carrier is utilized since thereference signal is transmitted through the air to a suitable receiver,yet to be described. The present transmitter may utilize any number ofcommercially available RF transmitters including, for example, spreadspectrum 2.5 GHz systems modulated with the reference signal. Theprinciples discussed above remain unchanged with the use of a higherfrequency reference signal. While the use of commercially available RFtransmitters is convenient, any suitable transmitter and/or carrierfrequency may be employed. One variation resides in generating thereference signal with a stable oscillator in the receiver/locator. Acooperating transmitter then periodically transmits a synchronizationsignal such as, for example, a synchronization pulse to lock theinternally generated reference signal of the receiver to the referencesignal that is in use in the transmitter. It is noted that in any RFembodiment, repeaters can be used to increase the range of the referencesignal so long as the phase of the repeated reference signal ismaintained or tracked for appropriate phase shifting in the receiver. Itis noted that using an RF carrier reference signal is advantageous atleast for the reason that the RF reference signal, transmitted throughthe air, is immune to higher attenuation rates which inground referencesignals may encounter, even though the carrier frequency for theinground reference signal can be significantly lower that the carrierfrequency of an RF reference signal. This feature is advantageous withrespect to long cables wherein an inground reference signal may becometoo weak to use, while the locating signal, being at a significantlylower frequency, remains strong.

Turning now to FIG. 10, another embodiment of a receiver, produced inaccordance with the present invention, is generally indicated by thereference number 104″. In this regard, it is noted initially thatreceiver 104″ is configured for use with a transmitter, such astransmitter 102″ of FIG. 9, having provisions for transmitting referencesignal R above ground using an RF carrier so as to extend thetransmission range to a distance that can be longer than the usefultransmission range of the locating signal transmitted through theground. Since receiver 104″ includes a number of components that areused in receiver 104 of FIG. 7, descriptions of these components willnot be repeated for purposes of brevity. Receiver 104″, however,includes telemetry arrangement 212 in the form of an RF receiver that isconnected to RF antenna 214 (see also FIG. 5) for receiving referencesignal R. RF receiver 212 is, in turn, connected to demodulator 274.Having recovered reference signal R, receiver 104″ operates inessentially the same manner as receiver 104 of FIG. 7.

Referring to FIGS. 9-11, the latter figure illustrates the use oftransmitter 102″ of FIG. 9 and receiver 104″ of FIG. 10 in region 10. Inparticular, transmitter 102″ is used to induce the locating signalfrequency onto cable 12 while transmitting reference signal R throughthe air for reception by receiver 104′.

Although each of the aforedescribed physical embodiments have beenillustrated with various components having particular respectiveorientations, it should be understood that the present invention maytake on a variety of specific configurations with the various componentsbeing located in a wide variety of positions and mutual orientations.Furthermore, the methods described herein may be modified in anunlimited number of ways, for example, by reordering, modifying andrecombining the various steps. Accordingly, it should be apparent thatthe arrangements and associated methods disclosed herein may be providedin a variety of different configurations and modified in an unlimitednumber of different ways, and that the present invention may be embodiedin many other specific forms without departing from the spirit or scopeof the invention. Therefore, the present examples and methods are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein.

What is claimed is:
 1. A cable locating system for use in a region whichcontains at least first and second in-ground cables, each of whichcables includes an electrically conductive component such that, when thefirst cable is driven at a locating signal frequency to emit a locatingsignal in an electromagnetic form, the locating signal can be coupled tothe second cable in a way which causes the second cable to generate afalse locating signal in electromagnetic form, said system comprising: atransmitter for generating said locating signal frequency for couplingonto the first cable and for transmitting a reference signal, whichincludes said locating signal frequency modulated thereon; and areceiver configured to distinguish the locating signal from the falselocating signal based on a modulation envelope of the reference signalas compared to the locating signal and the false locating signal.
 2. Thesystem of claim 1 further comprising: a coupler forming part of thetransmitter at least for coupling the locating signal frequency to thefirst cable such that the locating signal is subject to a coupling phaseshift; and a phase shifter forming part of a selected one of thetransmitter and the receiver for phase shifting the locating signalfrequency to at least approximately match the phase of the locatingsignal at least to compensate for the coupling phase shift.
 3. Thesystem of claim 2 wherein said coupler is configured to transmit thereference signal by coupling the reference signal onto the first cable.4. The system of claim 1 wherein the locating signal iselectromagnetically coupled from the first cable to the second cablethrough the ground to cause the second cable to emit the false locatingsignal subject to a phase shift between the locating signal and thefalse locating signal and wherein said receiver (a) recovers thelocating signal frequency from the reference signal, and (b)synchronously detects (i) said locating signal frequency, and (ii) aselected frequency band, which includes said locating signal and saidfalse locating signal to substantially reject the false locating signalwhile providing an output responsive to the locating signal.
 5. Thesystem of claim 1 wherein said locating signal frequency, as modulatedon the reference signal, is at least approximately in a fixed phaserelationship with said locating signal and distinguishing between thelocating signal and the false locating signal is based, at least inpart, on said fixed phase relationship.
 6. The system of claim 5 whereinsaid locating signal is driven onto the first cable using inductivecoupling and wherein said fixed phase relationship is at leastapproximately 90 degrees.
 7. The system of claim 5 wherein said fixedphase relationship is of a known magnitude and phase angle inrelationship to said locating signal frequency.
 8. The system of claim 7wherein said transmitter is configured for driving the locating signalonto said first cable in a given way which establishes said fixedfrequency relationship.
 9. The system of claim 1 wherein the transmitterproduces said reference signal including a carrier frequency that ismodulated by said locating signal frequency.
 10. The system of claim 9wherein a ratio of the carrier frequency to the locating signalfrequency is at least
 5. 11. The system of claim 9 wherein the locatingsignal frequency is in a range from approximately 1 Hz to 80 KHz and thecarrier frequency is in a range from approximately 10 Hz to 300 KHz. 12.The system of claim 1 wherein a phase shift between the locating signaland the false locating signal is in quadrature.
 13. The system of claim1 wherein the locating signal is electromagnetically coupled from thefirst cable to the second cable to cause the second cable to emit thefalse locating signal.
 14. The system of claim 1 wherein the first cableand the second cable each include a distal end and the distal ends areresistively connected to one another such that the locating signal isresistively coupled from the first cable to the second cable to producethe false locating signal in an inverted phase relationship with respectto the locating signal and said receiver indicates a signal strength ofone sign for the locating signal when an opposite signal strength isindicated for the false locating signal.
 15. The system of claim 1including a telemetry arrangement forming part of the transmitter andpart of the receiver for sending the reference signal from thetransmitter to the receiver.
 16. The system of claim 15 wherein saidtelemetry arrangement, at the transmitter, modulates an RF carrier withsaid reference signal and transmits the RF carrier above ground forreception by said receiver and said telemetry arrangement, at thereceiver, demodulates the RF carrier to recover the reference signal foruse at the receiver.